Numerous approaches are being employed to improve the bone generation and repair cycle (also referred to as the bone repair cascade). Such issues are paramount in the treatment of all bone related defects related to degeneration, injury, infection, malignancy or developmental malformation. For example, in the spinal surgery field, there are several different types of autologous bone graft substitutes that are either currently available or are in various stages of development for use in spine fusion surgery.
The use of bone grafts and bone substitute materials in orthopedic medicine is known. While bone wounds can regenerate without the formation of scar tissue, fractures and other orthopedic injuries take a long time to heal, during which time the bone is unable to support physiologic loading unaided. Metal pins, screws, rods, plates and meshes are frequently required to replace the mechanical functions of injured bone. However, metal is significantly more stiff than bone. Use of metal implants may result in decreased bone density around the implant site due to stress shielding. Physiologic stresses and corrosion may cause metal implants to fracture. Unlike bone, which can heal small damage cracks through remodeling to prevent more extensive damage and failure, damaged metal implants can only be replaced or removed. The natural cellular healing and remodeling mechanisms of the body coordinate removal of bone and bone grafts by osteoclast cells and formation of bone by osteoblast cells.
Conventionally, bone tissue regeneration is achieved by filling a bone repair site with a bone graft. Over time, the bone graft is incorporated by the host and new bone remodels the bone graft. In order to place the bone graft, it is common to use a monolithic bone graft or to form an osteoimplant comprising particulated bone in a carrier. The carrier is thus chosen to be biocompatible, to be resorbable, and to have release characteristics such that the bone graft is accessible. Generally, the formed implant, whether monolithic or particulated and in a carrier, is substantially solid at the time of implantation and thus does not conform to the implant site. Further, the implant is substantially complete at the time of implantation and thus provides little ability for customization, for example by the addition of autograft.
The use of bone grafts is generally limited by the available shape and size of grafts. Bone grafts using cortical bone remodel slowly because of their limited porosity. Traditional bone substitute materials and bone chips are more quickly remodeled but cannot immediately provide mechanical support. In addition, while bone substitute materials and bone chips can be used to fill oddly shaped bone defects, such materials are not as well suited for wrapping or resurfacing bone.
Demineralized bone matrix (DBM) is demineralized allograft bone with osteoinductive activity. DBM is prepared by acid extraction of allograft bone, resulting in loss of most of the mineralized component but retention of collagen and noncollagenous proteins, including growth factors. DBM does not contain osteoprogenitor cells, but the efficacy of a demineralized bone matrix as a bone-graft substitute or extender may be influenced by a number of factors, including the sterilization process, the carrier, the total amount of bone morphogenetic protein (BMP) present, and the ratios of the different BMPs present. DBM includes demineralized pieces of cortical bone to expose the osteoinductive proteins contained in the matrix. These activated demineralized bone particles are usually added to a substrate or carrier (e.g. glycerol or a polymer). DBM is mostly an osteoinductive product, but lacks enough induction to be used on its own in challenging healing environments such as posterolateral spine fusion.
Allograft bone is a reasonable graft substitute for autologous bone. It is readily available from cadavers and avoids the surgical complications and patient morbidity associated with harvesting autologous bone. Allograft bone is essentially a load-bearing matrix comprising cross-linked collagen, hydroxyapatite, and osteoinductive bone morphogenetic proteins. Human allograft tissue is widely used in orthopaedic surgery.
Even though allograft tissue has certain advantages over the other treatments, one of the main drawbacks of the allograft treatment is that the ingrowth of the host bone into the grafted bone may take longer than in an autograft. As a result, allograft treatment may be less effective than the autograft. Attempts have been made to overcome these drawbacks by modifying the bone graft's surface.
Despite the advances recently made in the art, new methods promoting ingrowth of the host bone into the grafted bone are needed to better utilize the advantages of allograft treatment.